Sandy shores spying

Posted: Friday, June 04, 2010

By Mary Willson

The rocky intertidal is often rich in species of many kinds. It is usually a popular habitat to visit for a class field trip or just a family outing. By comparison, sandy shores seem to provide poor fare for curious naturalists. Nevertheless, there can be much to see and contemplate in such places.

On a recent kayak junket to Berners Bay, as we ambled along the sandy beach in front of the Forest Service cabin, we noticed numerous bits of debris that moved in a seemingly purposeful way. Closer inspected solved this puzzle; under the debris bits were tiny hermit crabs, less than half an inch wide including the legs. Each one bore a very small periwinkle shell that protected its soft abdomen, at least partially.

According to one scientist who studies invertebrates, these miniature crabs were probably several molts away from their larval form, so they had been recognizably crab-like for some time. Eventually each one outgrows its borrowed shell and has to look for a new, larger one. If it has a shell that fits, it can pull itself back into the shell when threatened, and guard the entrance with its claws. At times, you may see fairly large hermit crabs toting a snail shell that is way too small to protect its soft parts, much less to allow the whole crab to retreat inside. This suggests that snail shells of the right sizes are not sufficiently abundant for all the hermit crabs, leaving some crabs very vulnerable to attack.

Sometimes, on this same beach I've seen hordes of young pink salmon less than two inches long, moving along the shallows. However, on this day, the action was a little way offshore, where Arctic terns were regularly capturing fish up to about four inches long. Juvenile herring, perhaps? And later, other watchers saw a humpback whale scooping them up.

A few days later, I took advantage of a nice minus tide to walk down the beach to the mouth of Ready Bullion Creek. It's far easier to go this way than through the woods! A flock of spotted sandpipers and a single semipalmated plover meandered along the waterline, foraging, and then took wing. Some of these birds may try to nest higher on the beach, amid the rocks and sticks and tufts of grass. Small barnacles, half buried in sand, waved their legs through shallow water in search of food particles.

A sea star had humped its body up over a shellfish that it was digesting. This sea star was purple, but we saw others, apparently of the same species, that were dressed in other colors, including orange and gray. I would love to know the biological significance of this color variation. Does it cost more, physiologically, to make one pigment rather than another? Does one color convey some advantage in one situation, while another color has an advantage in other circumstances? And if so, what could those advantages be?

A study of a similar sea star in more southerly waters found that the frequency of different color types varied with local ecology, particularly with the diet of the stars. If a certain species of mussel was lacking in the diet, the orange-ish colors did not develop. Furthermore, all of these sea stars tended to start their lives exhibiting the same nondescript color, but gradually changed as they grew. Some full-grown stars even changed their color. Other factors, in addition to diet, also may be involved, but remain to be explored. Here is a graduate thesis on the color variation of our local sea stars, ready-made for an interested student.

 Mary F. Willson is a retired professor of ecology. Special thanks to S. Tamone and G. Eckert contributing information for this column.